A burden, also often referred to as charge material, is fed into a blast furnace through a charging device arranged above the blast furnace. Such a charging device generally comprises one or more material hoppers for temporarily receiving the burden. The material hoppers are also used for weighing the burden contained therein and thereby control the amount of burden fed into the blast furnace.
During filling of the material hopper, the latter must be at atmospheric pressure. However, when the burden is fed into the blast furnace, the material hopper must be at blast furnace pressure. Therefore, the material hopper must be pressurized before the burden is transferred from the material hopper to the blast furnace.
This pressurization is generally carried out by feeding semi-clean top gas to the material hopper as shown in FIG. 1 and described amongst others in LU 73752. The blast furnace 10 comprises piping 12 for recovering top gas from a top section of the blast furnace. The recovered top gas is fed through a primary cleaning stage 14 and a secondary cleaning stage 16 before it is dried in a drying unit 18 and fed to a gas circuit 20. The secondary cleaning stage 16 comprises a primary prewashing and cooling stage 22 and a subsequent purification stage 24 wherein the gas is expanded. Semi-clean gas is extracted after the primary prewashing and cooling stage 22 and fed into a hopper chamber of a material hopper 26 for pressurizing the latter. Before the purification stage 24, the top gas is still at a relatively high pressure but must be compressed up to a pressure slightly above blast furnace pressure.
During the filling of the material hopper, air is drawn into the hopper chamber. When the material hopper is then sealed prior to pressurizing, the air gets trapped in the hopper chamber. The feeding of semi-clean gas into the hopper chamber forms a gas mixture comprising O2 from the atmospheric air and combustible gases CO and H2. In some cases, this gas mixture may occasionally lead to small deflagrations caused by impacting burden in the hopper. Such deflagrations should however be avoided as they may damage the material hopper.
In some cases, in particular in installations with higher CO and H2 concentrations, the risk of such deflagrations gets higher. This is in particular the case for top gas recirculation installations, wherein top gas is treated and a gas rich in CO and H2 is fed back into the blast furnace through the tuyere system. This inevitably leads to a higher concentration of CO and H2 in the material hopper and therefore to a higher risk of deflagrations. The risk of deflagrations is also increased if natural gas is injected in high quantities.
It should also be noted that attempts have been made in recent years to reduce CO2 emissions from blast furnaces so as to contribute to the general worldwide reduction of CO2 emissions. More emphasis has therefore been put on top gas recirculation installations wherein blast furnace top gas is fed to a CO2 removal unit wherein the CO2 content in the top gas is reduced, e.g. by Pressure Swing Adsorption (PSA) or Vacuum Pressure Swing Adsorption (VPSA), as for example shown in U.S. Pat. No. 6,478,841. PSA/VPSA installations produce a first stream of gas which is rich in CO and H2 and a second stream of gas rich in CO2 and H2O. The first stream of gas may be used as reduction gas and injected back into the blast furnace. The second stream of gas is removed from the installation and disposed of. This disposal controversially consists in pumping the CO2 rich gas into pockets underground for storage.
There is a need to provide an improved method for feeding a burden into a blast furnace, while avoiding deflagrations, in particular in view of the fact that top gas recirculation installations are becoming increasingly more popular.